Strange microphone problems when playing on certain Team Fortress 2 maps (Steam)

I’ve been having a problem using a mic on some team fortress 2 servers.
My mic works whether I test it in steam’s settings or in the game’s settings beforehand but the second I join some servers it no longer picks up, even if I go into the menu whilst connected to a game it suddenly doesn’t pick up the mic any more. If I hit shift+tab and test the mic in steam it still picks it up but that doesn’t help me much.

I’m running OpenSuSe 12.3 (loving it, great to come back to my first distro after so many years) currently but I’ve had the problem for the last few weeks trying it on ubuntu 12.04, 12.10 and 13.04.

The mic works perfectly for everything else. I have an ASUS Xonar d1 soundcard that I prefer running it through although I’ve tried running it through my on board sound as well and the problem persists.

Any help would be massively appreciated as I have not been able to find information on the problem anywhere on the web.